1. Field of the Invention
This invention relates generally to a process for recovering a metal in its elemental state from an oxide or salt of the metal by reacting the metal oxide or salt with a reducing gas and more particularly concerns a process for efficiently recovering a metal in its elemental state from an oxide or salt of the metal comprising passing the metal oxide or salt and reducing gas concurrently downward through a packed bed at a temperature above the melting point of the metal in its elemental state.
2. Description of the Prior Art
The recovery of a metal in its elemental state by the reaction of an oxide or salt of the metal with a reducing gas is well known in the art. A common technique for reducing a metal in its oxide or salt to its elemental state by reaction of the metal oxide or salt with a reducing gas is to perform the reduction in a fluidized bed at temperatures below the melting point of the elemental metal. However, a detrimental phenomenon that has been observed in the reduction of a metal to its elemental state in a fluidized bed at temperatures below the melting point of the elemental metal, is the tendency of the elemental metal to sinter and agglomerate, resulting in disruption of the fluidized state of the bed.
Stephens et al., U.S. Pat. No. 4,039,324 disclose a technique for the hydrogen reduction of copper in its oxide or salt to its elemental state, in a fluidized bed reactor, which circumvents the problem of sintering or agglomeration by employing a bed temperature of from about 200.degree. C. to about 1000.degree. C. and substantially chemically inert and generally spherical, relatively smooth, non-porous particles in the bed. Although agglomeration of the elemental copper to such a degree that defluidization of the bed occurs is prevented, the formation of the elemental copper in solid form necessitates a certain degree of agglomeration during which some bed particles are incorporated into particles containing the elemental copper and act as impurities therein. The resulting solid elemental copper is produced in the form of particles containing bed particles as well as copper. Upon completion of the fluidized bed reduction, the composite particles are removed from the reactor and further processed in order to separate the bed particles from the elemental-copper-containing particles. Thus this technique involves additional solids handling and separation aspects. Furthermore, Stephens et al. point out that, when the temperature at which the reduction is carried out exceeds 600.degree. C., copper is produced in the form of fines which are difficult to handle and separate from the fluidizing gas.
A technique which totally eliminates the problem of sintering and agglomeration and results in the production of an elemental metal in a highly pure state is disclosed in Reynolds et al., U.S. Pat. No. 4,192,676, and involves hydrogen reduction of a copper-bearing material at a temperature greater than the melting point of elemental copper under conditions which result in substantially instantaneous reduction coupled with efficient collection of the resulting elemental copper. Reynolds et al. point out that the resulting reduced copper particles are generally of the near sub-micron size and in liquid form and that collection of such particles is preferably accomplished as much as possible within the reactor. The preferred technique disclosed to effect the reduction and collection is the utilization of a cyclone flow pattern within the reactor. Such technique permits the small elemental copper particles to collect and coalesce into sufficiently large liquid particles in order to facilitate the copper recovery. Reynolds et al. disclose that other collection techniques that may be employed in lieu of or in combination with this cyclone technique include gravity settling in large chambers, wet scrubbing with collection of the copper as a powder cake, fabric filtering, and other known fine particle collection techniques.
Experience with the technique of Reynolds et al. indicates that efficient recovery of the resulting elemental copper often necessitates supplementing the collection of copper within the reactor with scrubbing of the off gases to collect the copper fines escaping from the reactor in the off gases. Furthermore, in the method of Reynolds et al., the copper oxide or salt must be introduced into the reactor in the form of a solid having a relatively small particle size. Thus, when dealing with feed components having a melting point less than the reaction temperature, it would be necessary in the method of Reynolds et al. to employ a technique which would maintain the feed in solid form until it is within the reaction vessel.
Other disadvantages of such prior art techniques are that fluidization and feeding a cyclone require high velocities and volumes of gas and that a great deal of energy is required to recycle the large volumes of gas because the fluidizing gas is generally cooled and then reheated in the recycling process.